The host-parasite relationship between several pathogenic rickettsiae and their natural arthropod vectors will be studied at the ultrastructural level. Electron microscopical investigations will be made of Rickettsia prowazeki in human body louse gut epithelium and R. prowazeki and R. mooseri in intestinal epithelium of the oriental rat flea. The findings obtained will be contracted with those of the microorganisms in mammalian cell culture as well as with observations of R. quintana propagated in the lumen of the body louse and on blood agar. We anticipate that these morphologic studies will help to explain why R. prowazeki produces a fatal infection in the louse while R. quintana does not and why neither R. prowazeki nor R. mooseri causes letal infection of the flea. An attempt will be made to correlate the ultrastructure of these rickettsiae in various metabolic states. Microorganisms in insect cells, gut lumen and infectious feces will be compared to those in infected tissue culture cells. Studies by freeze etch and freeze cleave procedures will be undertaken to elucidate structural details of the cell wall and plasma membrane. Finally, antibodies elicited to purified antigens of R. prowazeki and R. quintana will be labelled with electron dense markers and the antigenic sites localized on or in the rickettsiae.